Most small boats in private use today are taken to the water on specially designed boat trailers, from which they are launched and subsequently reloaded at sloping ramps or the like. To this end, boat trailers are equipped with a fixed roller assembly, and usually a winch and cable, to lift the boat from the water.
All boats have a sharp, forward V-section and usually a single centre keel which first contacts a central roller when reloading the boat. If the boat is out of line with the trailer, due to tide or wind, the keel may well roll off one end of the roller. The pull of the winch cable will tend to swing the boat to the centre, but this will only effectively come into play when the boat is some distance along the trailer. By that time of course, the boat may have been `lost`. It therefore becomes important to line up the boat carefully, with respect to the roller and hence the trailer, before starting the winch, to ensure the boat does not run off the roller.
There are times, though, when conditions are such that accurate alignment is well nigh impossible: very choppy seas, for instance. Modifications have been made to the design of the roller proper with this in mind and with an eye on minimizing the possibility of `losing` the boat. For example, the continuous surface of the roller, which is normally approximately concave, may be modified so as to be more steeply inclined to either side of the middle of the roller. This, however, can introduce another problem, namely jamming of the boat keel. Also, the ends of the roller can be altered to include stops designed to prevent the boat riding over the ends, but this arrangement adds substantially to the cost of the assembly as a whole and, besides, is not especially effective for the job at hand.